Power MOS devices, including lateral diffused MOS (LDMOS) devices, are employed in a wide variety of applications, such as, for example, power amplifiers in wireless communications systems. HCI degradation can significantly limit the performance of these devices, particularly when operating in a saturation region. As is well known, the HCI phenomenon generally results from heating and subsequent injection of charge carriers into the gate oxide and/or an oxide layer above a drift region of an LDMOS device. This injection of charge carriers often results in a localized and nonuniform buildup of interface states and oxide charges near and underneath a gate and/or in the drift region of the device. It has been shown that, over time, several electrical characteristics of the LDMOS device degrade as a direct result of HCI (see, e.g., S. Manzini et al., “Hot-Electron-Induced Degradation in High-Voltage Submicron DMOS Transistors,” Proc. IEEE ISPSD, pp. 65-68, 1996, which is incorporated by reference herein). For example, HCI can produce variations in certain characteristics of the LDMOS device, including saturation current, threshold voltage, transconductance, on-resistance, etc., thus undesirably affecting the performance and reliability of the device.
Studies of LDMOS HCI degradation demonstrate a significant increase in on-resistance without a significant change in a threshold voltage of the device as a result of HCI (see, e.g., D. Brisbin, et al., “Hot Carrier Reliability of N-LDMOS Transistor Arrays for Power BiCMOS Applications,” Proc. IEEE IRPS, pp. 105-110, 2002, and P. Moens, et al., “A Unified Hot Carrier Degradation Model for Integrated Lateral and Vertical nDMOS Transistors,” Proc. IEEE ISPSD, pp. 88-91, 2003, which are incorporated by reference herein). This is more typical for an LDMOS device since the threshold voltage is determined primarily by the peak concentration of doping in a channel region of the device, close to a source region of the device. Saturation current generally decreases in the LDMOS device as a result of HCI.
A conventional LDMOS device typically includes a lightly-doped drain (LDD) region which is often formed at or near an upper surface interface between the silicon and oxide of the device. Locating the LDD region in close relative proximity to the silicon/oxide interface, however, significantly increases the likelihood that charged carriers will become trapped at the interface, thereby increasing HCI degradation in the device. The amount of HCI degradation in the device can be measured as a function of the amount of increase in the on-resistance of the device (on-resistance degradation) and/or the amount of decrease in the saturation current (saturation current degradation) in the device.
In many applications, such as, for example, power applications, it is desirable to minimize the on-resistance associated with the device. In an LDMOS device, since the on-resistance is dominated primarily by the characteristics of the LDD region, one known methodology for reducing the on-resistance is to increase a doping concentration of the LDD region. However, since the LDD region is typically formed at the silicon/oxide interface of the device, as previously stated, increasing the doping concentration of the LDD region also undesirably increases HCI degradation in the device. The increase in HCI degradation resulting from the increased doping concentration of the LDD region often significantly undermines any beneficial reduction in on-resistance that may otherwise be achieved by increasing the doping concentration of the LDD region. Furthermore, by increasing the doping concentration of the LDD region in the device, a breakdown voltage of the device is undesirably reduced.
There exists a need, therefore, for an improved MOS device capable of controlling HCI degradation that does not suffer from one or more of the problems exhibited by conventional MOS devices. Moreover, it would be desirable if the improved MOS device were compatible with existing integrated circuit (IC) fabrication process technologies.